Prior study has provided data suggesting that interferon gamma 1b improves ne resorption, yielding an increase in bone marrow space; improves white cell super-oxide production, and reduces the trend toward progressive narrowing the cranial nerve foramina. This study proposes to determine if interferon gam 1b therapy combined with calcitriol can improve bone turnover, marrow function and the size of cranial nerve foramina in severe osteopetrosis to a greater ext t than calcitriol alone through a randomized, controlled, open-label trial.